Copper when dispersion strengthened by oxides (Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2 and BeO), borides (TiB.sub.2) and carbides (ZrC, TaG and NbC) offers a unique combination of high strength and hardness with excellent electrical conductivity. Such alloys are used for the making of lead frames and spot welding electrodes. A good discussion of desirable lead frame characteristics is contained in U.S. Pat. No. 4,732,733 to Sakamoto, et al. The effectiveness of the dispersed particles in the matrix strengthening and the strength retention depends upon the particle size and interparticle spacing. The primary requirements to create a homogeneous dispersion of a second or hard phase are: (a) small particle size, (b) low interparticle spacing c) chemically stable second phase. The refinement of the second phase is very important according to Orowan's strengthening mechanism .tau.=.mu.b/L, where .tau. is the external stress, .mu. the modulus, b the Burgers vector and L the spacing of particles. Transition nitrides such as TiN and ZrN possess high electrical conductivity, high hardness and a high melting point. It is expected that a copper alloy dispersion hardened by TiN and ZrN would be extremely useful. However, little research has been done on the copper dispersion hardened by TiN and ZrN.
Previously, only TiN or ZrN surface films resulted when bulk copper alloys were exposed to a nitrogen environment at elevated temperatures. This is because of the insolubility of nitrogen in copper for both liquid and solid states. This method is described in U.S. Pat. No. 5,096,508 to Breedis, et al.
Therefore, in order to prepare a TiN dispersion hardened copper alloy a powder metallurgical method in combination with external nitridation was conceived. The idea of this mechanical alloying procedure was to break down the surface nitride of the alloy powder formed during nitridation in order to produce a homogeneous distribution of nitride in the matrix. A similar method has been used successfully to break down the surface Al.sub.2 O.sub.3 on aluminium powder to fabricate aluminium alloy dispersion hardened by Al.sub.2 O.sub.3. This is taught by the publication to J. S. Benjamin and M. J. Bomford in Metallurgical Transaction A, Vol. 8A, Aug. 1977,P1301-P1305. Compared with the conventional approach of directly mixing TiN and Cu powder, this procedure has the following advantages: (a) TiN is very uniformly distributed after nitridation process. The function of mechanical alloying is just to break down the thin TiN layer. (b) The surface layer thickness of TiN can be controlled by the size of the copper alloy powder. For the conventional alloying, many researchers have found it is not very efficient to crush down hard phases such as TaC and ZrB.sub.2. Particle coarsening can usually be given by the equation r.sup.3 -r.sub.o.sup.3 =8V.sup.2 .gamma.DC.sub.i t/9R, where r and r.sub.o are the particle radius at time t and initially, V is the molar volume of the precipitate, .gamma. the particle/matrix surface energy, C.sub.i is the solubility of the particle and D is the diffusion constant of the rate limiting constituent. Thus to obtain a low coarsening rate and good thermal microstructural stability, the product of .gamma. DC.sub.i needs to be small. In our case, because nitrogen is virtually insoluble in copper, the diffusion of nitrogen from small particles of nitrides to large particles through the Cu matrix is virtually impossible. That means the small nanophase nitride particles should be stable even at high temperatures approaching the melting temperature.